1. Field of the Invention
This invention relates to high temperature fuel cells having metal-containing anode electrodes, in particular, solid oxide fuel cells and, more particularly, to solid oxide fuel cell anode electrodes. More particularly yet, this invention relates to solid oxide fuel cell anode electrodes that are redox tolerant, solid oxide fuel cells comprising such electrodes, and a method for enhancing the redox tolerance of such electrodes.
2. Description of Related Art
Fuel cells are electrochemical devices that convert the chemical energy of a fuel into electrical energy with high efficiency. The basic physical structure of a fuel cell consists of an electrolyte layer with a porous anode electrode and porous cathode electrode on opposed sides of the electrolyte. In a typical fuel cell, gaseous fuels, typically hydrogen, are continuously fed to the anode electrode and an oxidant, typically oxygen from air, is continuously fed to the cathode electrode. The electrochemical reactions take place at the electrodes to produce an electric current.
In a solid oxide fuel cell, the electrolyte is a solid, nonporous metal oxide, normally Y2O3-stabilized ZrO2 (YSZ), the anode electrode is a metal/YSZ cermet and the cathode electrode is typically Sr-doped LaMnO3. The solid oxide fuel cell operating temperature is typically in the range of about 650° C. to about 1000° C., at which temperature ionic conduction by oxygen ions occurs.
The most commonly used solid oxide fuel cell anode material is a porous two phase nickel and yttria stabilized zirconia (Ni/YSZ) cermet. During normal fuel cell operation, this anode material remains a cermet. However, there are potentially several occurrences, such as air leakage into the anode side of the fuel cell due to seal leakage, fuel supply interruption, and emergency stops, which may cause the anode electrode to re-oxidize, forming an NiO/YSZ structure. Upon restarting of the fuel cell, the NiO/YSZ structure is chemically reduced to re-form the Ni/YSZ anode electrode. However, this reduction and oxidation process (referred to as redox cycling) results in substantial bulk volume changes of the anode electrode. The bulk volume of a fully dense NiO sample would be expected to contract by about 40.9% upon reduction and would be expected to expand by about 69.2% upon oxidation. Although, due to expansion into the pores, a NiO/YSZ solid oxide fuel cell anode electrode is unlikely to experience such a drastic volume change, any volume change that does occur can have a significant effect on the integrity of other cell components (e.g. electrolyte cracking) and cell component interfaces, which can, in turn, result in a significant degradation in the performance of the fuel cell.